1 Control Strategies for MMC Using Cells with Power Transfer Capability Mario L´ opez, Fernando Briz, Alberto Zapico, Alberto Rodr´ ıguez and David Diaz-Reigosa Department of Electrical Engineering, University of Oviedo, Spain lopezmario@uniovi.es Abstract—Conventional MMCs use cells which typically consist of a half-bridge and a capacitor. Due to their limited energy storage capability, the net power balance of the cells is zero (neglecting losses), the MMC therefore realizing a bidirectional power transfer between its DC and AC ports. It is possible how- ever to provide the MMC with the capability to transfer power at the cell level. The use of such cells opens new functionalities and uses for the MMC, including integration at the cell level of distributed energy storage (e.g. batteries), low-voltage/low power sources/loads, and its operation as a multiport power converter, combining high and low voltage AC and DC ports. Existing control strategies for MMCs assume that all the cells have an identical design and operate identically. However, use of cells with power transfer capability can result in imbalances in their operation, provided that not all the cells transfer power, or that they do not transfer the same amount of power. This paper addresses the design and control of MMCs using cells with power transfer capability, with special focus on the design of suitable control strategies and on the definition of their limits of operation. Index Terms—Modular Multilevel Converter, MMC, Multiport Power Converters, Solid State Transformer I. I NTRODUCTION Reducing the dependence on conventional fossil fuels has become a priority for industrialized countries due to environ- mental concerns, limited resources and the progressive increase of their cost. This scenario has pushed the penetration of re- newable energies in the existing transmission system. However, massive integration of renewable energy into the existing and future grids poses major challenges, as a significant part of the installed capacity will be connected to the distribution levels [1]. Innovative solutions based on high-power, high-voltage electronic power converters, like High Voltage Direct Current (HVDC) and Flexible AC Transmission Systems (FACTS) have the potential to cope with these challenges, also providing to the power system operator functionalities such as power flow control, power quality improvement and reduction of transmission losses among others [2]. Multilevel converters are well suited for medium-high volt- age/power ranges which are required for electronic power converters connected to medium voltage electrical grids [3]. Among these, the Modular Multilevel Converters (MMCs) This work was supported in part by the Research, Technological Devel- opment and Innovation Programs of the Spanish Ministries of Science and Innovation and of Economy and Competitiveness, under grants MICINN- 10-CSD2009-00046 and MINECO-13-ENE2013-48727-C2-1-R, and by the European Commission FP7 Large Project NMP3-LA-2013-604057, under grant UE-14-SPEED-604057. appear as a promising topology for applications requiring a high voltage DC port (e.g. HVDC), being a hot research topic nowadays. MMC was first introduced one decade ago [4]-[6]. It realizes a bidirectional DC/AC power conversion, sharing the advantages of other multilevel converters: reduced size of filters due to better output voltage wave shape; lower switching losses due to the reduced switching frequency; capability of withstanding large terminal voltages using rel- atively low voltage power devices. Additionally, it provides attractive features compared to other multilevel topologies, such as modularity (identical cells are piled-up to increase the voltage) and consequently easy scalability, and distributed energy storage, therefore eliminating the need of a bulk DC capacitor [4]-[7]. Conventional MMC use cells consisting of a half-bridge and a capacitor. Control and modulation strategies developed for MMCs are aimed to balance the power between the AC and DC ports, which is needed to maintain the average voltage of the cells capacitors at its target value. This is done by controlling the circulating current either explicitly [8]-[13] or indirectly (i.e direct modulation) [14]-[16]. Balancing of the cell capacitor voltages is also required [4]-[16]. Due to the fact that the cells have a limited energy storage capability, the net power balance for each cell is zero (neglecting losses), AC and DC powers being therefore equal to each other. It is possible however to transfer (absorb or deliver) power through the MMC cells. This would provide the MMC with new potential features, including distributed energy storage [17]; integration of distributed energy resources (DER) at the cell level; multiport multilevel power converters combining the medium/high voltage DC and AC ports of the MMC with low voltage DC and AC ports. Another potential application being Solid State Transformers (SST) [19]-[21]. This paper addresses the design and control of MMCs using cells with power transfer capability. The paper is organized as follows. Basic concepts and power balance requirements of conventional MMCs are presented in Section II. Section III extends the analysis to the case of MMCs using cells with power transfer capability. Section IV presents MMC config- urations using cells with power transfer capability. Limits of operation and control strategies are presented in Sections V and VI respectively, simulation and experimental results being presented in Sections VII and VIII. Finally, conclusions are presented in Section IX. 978-1-4673-7151-3/15/$31.00 c  2015 IEEE